Regulated power supplies are found in virtually all electronic devices, including battery chargers, cellular telephones, computers, computer monitors, televisions, audio equipment, and video cameras. One typical power supply, the DC to DC converter, operates from a DC source, generates an alternating current as an intermediate process, and delivers direct current to a load. Switched DC converters, like linear, transformer-based power supplies, deliver regulated output and provide isolation between input and output circuits. Unlike a linear power supply, however, the transformers in a switched DC converter operate at much higher frequencies, frequencies as high as several megahertz. Such operation permits the use of small value components, including transformers and capacitors, while still providing for complete isolation between the input and the output of the converter.
Although widely used in electronic and computer applications, switched DC converters are known to introduce radiated losses and RF noise during switching. In hard-switching topologies for example, switching causes a rapid transition in the current through or voltage across the converter's switch and results in switching losses in the form of signal emissions. These switching transients have a spectrum containing high frequency components, appear as radiated emissions, and introduce noise into the area surrounding the converter. Such noise may interfere with video signals or the like, thus requiting additional processing and filtering of transmitted signals to obtain a more useful, noise-free form.
Soft-switching converters present a known converter topology for reducing hard switching losses and associated noise-related problems. A typical soft-switching converter utilizes a resonator to force either the current or voltage in a power switch to zero during switching. This action significantly reduces switching transients and radiated emissions caused by rapid transitions in the switching current and/or voltage.
Soft-switching converters, however, also fail to achieve optimum efficiency. For example, in a display device, the regulated power supply typically operates in various modes. In a primary mode, all the components of the display monitor are powered, and collectively dissipate between 50 W and 100 W in displaying screen images which range from all black to all white. In a standby mode, a minimal number of display monitor components, such as the power supply, microprocessor and remote control circuitry, are powered. Standby mode has a constant load characteristic that typically consumes less than 5 W. In a suspend mode, an intermediate mode between the primary and standby modes, several but not all components of the display monitor are active. Approximately 10 W to 20 W are dissipated by a display device in the suspend mode.
To provide low power operation (less than 20 W), typical soft-switching power supplies reduce their switching frequency, since only a small amount of power must be delivered to a load. This method of providing low power results in erratic feedback for the zero voltage switching detection unit, and introduces an unacceptable power loss in the switch of such a power supply. In addition, typical soft-switching power supplies suffer intolerable constraints in the range of power available for delivery in low power operating modes.